Our working hypothesis is that Paget's disease of bone is analogous to a slow virus infection associated with a virus of the paramyxoviridae family. In order to test this hypothesis it will be necessary to conduct a series of experiments to extend our preliminary data which detected viral antigens to respiratory syncytial (RS) and measles viruses in Paget's lesions of bone. The first task will be to identify the putative virus by specific immunofluorescent and immunoperoxidase staining of paget's bone and bone cells derived from Paget's bone in cell culture using monoclonal antibodies to RS virus and measles virus. Once the virus or viruses are identified and the defect recognized by use of the specific probes, confirmation will be sought by use of immunoprecipitation of the viral proteins. If these methods are too specific and do not recognize the antigen present in Paget's bone, which might be different, peptide separation will allow comparison of Paget's cell peptides with authentic RS virus or measles virus peptides following density gradient separation of the viruses, and gel electrophoresis of the peptides. By means of this technique a band unique to Paget's antigen might be discovered by comparison with uninfected normal human adult bone derived cells. If the virus remains undetectible because it is present in such minute quantities, a specific and highly sensitive method will be used. Hybridization in situ can detect as little as 1 to 10 copies of a gene product. This procedure, testing for measles virus and RS virus will be employed using antigen positive Paget's bone cells by A. Haase, MD. in collaboration. While the above identification process is proceeding, attempts to rescue the putative virus will be made monitoring the process with hyperimmune serum using immunohistochemistry by our published techniques and by electron microscopy. Attempts to produce bone lesions in Nude mice will continue using the transformed cells recently obtained from cultured Paget's bone and a giant cell tumor arising in Paget's bone. When these studies are completed the pathophysiology of Paget's disease may be known.